This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to packaging of integrated circuits.
Integrated circuits are preferably packaged prior to use in a larger circuit, so as to protect the integrated circuit and provide for electrical interconnections with other parts of the larger circuit that are more easily made. Different types of packaging are used for different types of integrated circuits. For example, one type of integrated circuit is classified as a flip chip. Flip chips are distinguishable from other types of integrated circuits in that the electrical contacts for the integrated circuit, or bonding pads, are typically distributed across the entire top surface of the integrated circuit, rather than being limited to a few rows around the peripheral edge of the integrated circuit, as is done with some other integrated circuit types.
Flip chip integrated circuits are typically packaged by attaching them with solder bumps to a package substrate. Electrical connections to an outside circuit are made through the package substrate, which reduces the concern that the integrated circuit may be damaged, such as by further handling. The mounted flip chip is under filled with a material to enhance the structural strength of the integrated circuit and package substrate combination.
Two other elements are typically added to a flip chip package. The first is a stiffener, which is used to increase the structural strength of the package substrate. The package substrate is often a relatively thin, flexible structure. If the package substrate is allowed to flex to an excessive degree after the integrated circuit is mounted, the mechanical stresses inherent in the flexation may be transferred to and damage the integrated circuit. Typically, the stiffener is a piece of relatively rigid material, such as a flat piece of metal, that has a square or rectangular hole cut in the middle of it. The stiffener is attached to the package substrate, with the integrated circuit disposed in the square cut out of the stiffener.
The second additional element is a heat spreader, which is used to more efficiently remove thermal energy from the integrated circuit. When the integrated circuit is in use, it tends to generate thermal energy as a by product of the electrical activity within the integrated circuit. If the thermal energy is not adequately dissipated, it tends to build up and be expressed by a temperature increase in the integrated circuit. If the thermal energy is not sufficiently dissipated, then the temperature increase may reach a level that is damaging to the integrated circuit. The heat spreader is typically a structure such as a lid, which is connected to both the package substrate and the integrated circuit. The connection between the integrated circuit and the heat spreader typically exhibits good heat conduction properties. In addition, the heat spreader typically conducts thermal energy well, and is formed of a material such as a metal.
Unfortunately, there are often problems associated with these two elements of the packaged integrated circuit. For example, in regard to the stiffener, it is desirable to have it come close to the corners of the integrated circuit, but reducing the size of the cut out tends to create problems with the under fill material that is exposed at the sides of the integrated circuit. In regard to the heat spreader, it is easily misaligned when attached to the package substrate, thus creating problems with the reliability of the packaged integrated circuit.
What is needed, therefore, is package design that overcomes these and other problems with integrated circuit package designs.
The above and other needs are met by a packaged integrated circuit according to the present invention. The packaged integrated circuit includes a package substrate having electrical contacts for receiving an integrated circuit. The integrated circuit is electrically connected to the electrical contacts of the package substrate. A stiffener is mounted to the package substrate, where the stiffener has a non-orthogonal cut out in which the integrated circuit is disposed. The edges of the cut out are disposed at no greater a distance from the corners of the integrated circuit than they are from the sides of the integrated circuit.
Thus, in this manner the stiffener provides enhanced structural support to the packaged integrated circuit, because the edges of the cut out are disposed at no greater a distance from the corners of the integrated circuit than they are from the sides of the integrated circuit. In a typical stiffener design with an orthogonal cut out, the edges of the cut out are disposed at a greater distance from the corners of the integrated circuit than they are from the sides of the integrated circuit. This inhibits the prior art stiffener from providing the enhanced structural support at the corners of the integrated circuit that is provided by the stiffener of the present invention.
However, the stiffener of the present invention provides this enhanced support at the corners of the integrated circuit without having the edges of the cut out encroaching closer to the sides of the integrated circuit, which would tend to interfere with the under fill of the integrated circuit. In a prior art stiffener design, the edges of the cut out would have to encroach on the sides of the integrated circuit in order to get closer to the corners of the integrated circuit. Thus, the packaged integrated circuit according to the present invention has benefits over the prior art.
In various preferred embodiments, a heat spreader is thermally connected to the integrated circuit. Preferably, the edges of the cut out are disposed at a lesser distance from the corners of the integrated circuit than they are from the sides of the integrated circuit. In one embodiment, the stiffener extends to a peripheral edge of the package substrate and the heat spreader is mounted within a recessed portion at a peripheral edge of the stiffener. In an alternate embodiment the stiffener does not extend to the peripheral edge of the package substrate and the heat spreader is mounted within a recessed portion at the peripheral edge of the package substrate.
In yet another embodiment, the stiffener does not extend to the peripheral edge of the package substrate and the heat spreader is mounted at the peripheral edge of the package substrate. In a further embodiment, the stiffener does not extend to the peripheral edge of the package substrate and the heat spreader is mounted at the peripheral edge of the package substrate and is electrically connected to a ground contact of the package substrate.
Alternately, the stiffener extends to a peripheral edge of the package substrate and is electrically connected to a ground contact of the package substrate, and the heat spreader is mounted within a recessed portion at a peripheral edge of the stiffener and is electrically connected to the stiffener and is thus in electrical contact with the ground contact of the package substrate. Preferably, the stiffener and the heat spreader are formed of copper. In one embodiment the cut out of the stiffener is formed in a circular shape, and in another embodiment the cut out of the stiffener is formed in an octagonal shape. Most preferably, the edges of the stiffener are disposed at a lesser distance from the corners of the integrated circuit than they are from the edges of the integrated circuit.